Torque limiting apparatus are known in which the input shaft is prevented from rotating if the torque in the system becomes excessive due to mechanical overloading. These apparatus are used to protect mechanical equipment from damage caused by excessive torque.
A known type of torque limiting apparatus comprises a torsion bar that transmits the turning force (torque) from the input shaft to the output shaft, so that rotation of the input shaft drives a corresponding rotation of the output shaft. If there is excessive resistance to rotation of the output shaft due to mechanical overloading, then the input shaft forces the torsion bar to twist, enabling the input shaft to rotate relative to the output shaft. The torque limiting apparatus is configured such that this relative rotation urges rollers, which are coupled to the input shaft, into contact with a surrounding casing, thereby preventing rotation of the input shaft relative to the casing and hence preventing further torque being applied to the output shaft.